Recently, water treatment in a membrane method, which uses a filtration membrane which is excellent in separation ability and compactness, is drawing attention in view of increasing interest in environmental pollution and tightening of regulations. In order to use in such water treatment, a filtration membrane is demanding to have not only excellent separation property and permeation capacity, but also mechanical properties which are higher than those of ordinal filtration membranes.
As conventional filtration membranes having excellent permeation capacity, filtration membranes made of polysulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, and the like, which are produced by a wet spinning method or a dry and wet spinning method, have been known. These filtration membranes are produced by micro-phase separation of a polymer solution and solidification of the polymer solution in a non-solvent. These filtration membranes comprise a dense layer and a support layer, and they have high pore ratio and an asymmetrical structure.
Among materials for these filtration membranes, polyvinylidene fluoride resin is suitably used for material of a separation membrane, since it has excellent chemical resistance and heat resistance. However, there is a problem that a filtration membrane made by polyvinylidene fluoride hollow fiber membrane has inferior mechanical strength.
A porous membrane in which hollow braids are entirely embedded in a porous semipermeable membrane has been suggested as a membrane having improved strength. (Japanese Unexamined Patent Applications, First Publication Nos. S52-081076, S52-082682, and S52-120288). However, since the braids are entirely embedded in the porous semipermeable membrane, this porous membrane has a problem in water permeation capacity.
In order to improve water permeation capacity, a separation membrane, in which a porous membrane is provided on the surface of hollow braids, has been suggested (U.S. Pat. No. 5,472,607). However, since this separation membrane has a porous membrane only on the surface of the braids, this has a problem that the porous membrane is easily peeled from the braids.
In consideration of the above-described problems, an object of the present invention is to provide a composite porous membrane, which has not only excellent filtration capacity, but also excellent adhesion between a porous membrane and a braid and mechanical properties, and a method for producing the composite porous membrane.